Silver nanoparticles are widely used in various sciences and industries. Widespread use of silver nanoparticles has caused entrance of it to the aquatic environment through Wastewater, so it is necessary to investigate the effects of silver nanoparticles on various aquatic organisms. The present study investigated the lethal concentration and bioaccumulation of silver nanoparticles and its comparison with silver nitrate in the Cas k pian Sea gammarus (Pontogammarus maeoticus). For this purpose, the first experiment was performed to determine the range finding test of silver nanoparticles in gammarus. Then, to determine the lethal concentration 50 (LC50), the gammarus were exposed to concentrations of 0, 0.25, 0.5, 1, 2, 4 and 8 mg/L of silver nanoparticles and silver nitrate and their mortality was recorded in 96 hours. Then different lethal concentration were calculated by statistical analysis. The bioaccumulation of silver in each of the silver nanoparticles and silver nitrate treatments was also investigated. In this study, the 96-h LC50 values ​​for silver nanoparticles and silver nitrate were 3.77 and 5.78 mg/l, respectively .The results of this study showed that the LC50 of silver nanoparticles was lower than silver nitrate and the gammarus mortality increased with increasing concentrations of silver nanoparticles and silver nitrate as well as increasing exposure time.